Research ArticleA Novel Simple Phantom for Verifying the Dose ofRadiation Therapy

J. H. Lee,1 L. T. Chang,1 A. C. Shiau,2,3 C. W. Chen,4,5 Y. J. Liao,6 W. J. Li,5

M. S. Lee,7 and S. M. Hsu3,8

1 Health Physics Division, Institute of Nuclear Energy Research, Longtan 325, Taiwan2Department of Radiation Oncology, Koo Foundation Sun Yat-Sen Cancer Center, Taipei 112, Taiwan3Department of Biomedical Imaging and Radiological Sciences, National Yang-Ming University, Taipei 112, Taiwan4Department of Anesthesiology, China Medical University Hospital, Taichung 404, Taiwan5 Graduate Institute of Clinical Medical Science, China Medical University, Taichung 404, Taiwan6 School of Medical Laboratory Science and Biotechnology, Taipei Medical University, Taipei 110, Taiwan7 School of Medicine, Tzu Chi University, Hualien 970, Taiwan8 Biophotonics and Molecular Imaging Research Center, National Yang-Ming University, Taipei 112, Taiwan

Correspondence should be addressed to S. M. Hsu; smhsu@ym.edu.tw

Received 3 June 2014; Revised 21 October 2014; Accepted 22 October 2014

Academic Editor: Tsair-Fwu Lee

Copyright © 2015 J. H. Lee et al.This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A standard protocol of dosimetric measurements is used by the organizations responsible for verifying that the doses delivered inradiation-therapy institutions are within authorized limits.This study evaluated a self-designed simple auditing phantom for use inverifying the dose of radiation therapy; the phantom design, dose audit system, and clinical tests are described.Thermoluminescentdosimeters (TLDs) were used as postal dosimeters, and mailable phantoms were produced for use in postal audits. Correctionfactors are important for converting TLD readout values from phantoms into the absorbed dose in water. The phantom scattercorrection factor was used to quantify the difference in the scattered dose between a solid water phantom and homemade phantoms;its value ranged from 1.084 to 1.031. The energy-dependence correction factor was used to compare the TLD readout of the unitdose irradiated by audit beam energies with 60Co in the solid water phantom; its value was 0.99 to 1.01. The setup-condition factorwas used to correct for differences in dose-output calibration conditions. Clinical tests of the device calibrating the dose outputrevealed that the dose deviation was within 3%. Therefore, our homemade phantoms and dosimetric system can be applied foraccurately verifying the doses applied in radiation-therapy institutions.

1. Introduction

Cooperation between the International Atomic EnergyAgency (IAEA) and the World Health Organization (WHO)led to the implementation of the IAEA/WHO thermolumi-nescent dosimeter (TLD) postal program for verifying thequality of radiotherapy treatment units. Worldwide TLD-based quality assurance networks confirmed the benefits ofthis program between 1969 and 1998 [1–4]. In 2007 the IAEAalso designed a horizontal arm for use with the originallydesigned holder for off-axis measurements [5]. In 1997,ESTRO organized a new department, EQUAL, to send TLDs

and the IAEA standard holder to participating radiotherapytreatment units in Europe and the Middle East [6]. Theorganization responsible for radiotherapy quality assurancein the United States is the Radiologic Physics Center (RPC),which is a section of department of theMDAndersonCancerCenter at the University of Texas.

In recent years, the RPC has developed some mail-able anthropomorphic dosimetry phantoms for use in dif-ferent radiotherapy techniques, such as prostate intensity-modulated radiation therapy (IMRT), head and neck IMRT,stereotactic body radiotherapy for the liver and thorax, andstereotaxic radiosurgery [7]. During this time, the number of

Hindawi Publishing CorporationBioMed Research InternationalVolume 2015, Article ID 934387, 5 pageshttp://dx.doi.org/10.1155/2015/934387

2 BioMed Research International

110mm90mm

90mm

90mm

100mm

100mm

110mm

95mm

5mm

5mm5mm

(a)

5mm

2mm2mm10mm

10mm

30mm

30mm

(b)

Figure 1: (a) Self-designed simple phantom. (b) Dosimeter hole in homemade phantoms.

the participating organizations had increased. The RPC hasalso provided criteria for acceptable compliance such as theabsorbed dose of external beams and brachytherapy and themechanical parameters of linear accelerators (LINACs). Themost important criterion has been for the delivered tumordose to be within 5% of the target dose.

Multipurpose phantoms have also been designed inseveral countries (e.g., the Czech Republic and Poland) formeasuring multiple parameters simultaneously [8–14]. Theaim of the present study was to design a lightweight, low-workload, easy-to-set-up, and transportable phantom for usein mailed TLD postal audit programs. This was achieved byconsidering dosimeter selection, dose-response calibration,phantom scatter correction, and energy-dependent correc-tion.

2. Material and Methods

The primary experiment of this study involved verifying thedose outputs of the photon and electron beams producedby medical LINACs. The cubic TLDs (TLD-100) were usedin this study. A 60Co source established at the Instituteof Nuclear Energy Research (INER, Taiwan) was used toirradiate the dosimeters. The dosimeter remote-monitoringsystem was tested at the INER for accuracy, stability, andenergy dependence.

2.1. Phantom Design. Various dosimeters or phantoms havebeen developed for postal audit programs in recent decades.Such mailable audit systems have been designed to be inex-pensive, lightweight, and easy to set up precisely. Nonetheless,these phantoms also have some disadvantages, such as theinconvenience of a water tank being involved with the IAEAholder and the sizes and measuring depths of RPC phantomschange with the beam modality and beam energy.

With the aim of improving the efficiency and accuracy ofmailable phantoms, we designed an acrylic phantomwith fea-tures of smallness, simple geometry, and depth adjustment.

The dimensions of this phantom are 110 × 110 × 100mm3(Figure 1(a)), with its internal space containing three 90 ×90 × 30mm3cubic layers. One layer contains a hollowed-out 30 × 30 × 30mm3 hole to allow the TLD columnsto be changed. The TLD columns are divided into twoparts. For the direction of irradiation, the thickness of theupper part depends on the measuring depth; the other partcontains three circular and three rectangular holes in whichdosimeters can be placed, as shown in Figure 1(b). In orderto increase the accuracy of the experiment, we can obtainthe six TLD readings in a single irradiation. Although someaspects such as a wide field measurement range and depthvariation are not compatible with small acrylic phantoms,our phantom offers a solution to the depth limitation byproviding the ability to adjust the three layers in the differentdirection.

2.2. TLD Calibration Procedures. When developing a dose-verification system, we needed to know which TLDs aresufficiently stable to use as postal dosimeters. There arevarious types of TLD available, which makes it difficult toensure linearity between responses and doses. As a first step,we used aTLD reader (Harshaw5500) to identifywhichTLDsin the same batch were unstable. By delivering doses of 80,160, 240, and 80 cGy using the 60Co source at the INER, wecompared the batch average response with the response ofeach TLD for the same dose.

A dose-response curve is needed to convert the TLDresponse into the absorbed dose after irradiation.We dividedthese stable TLDs (coefficient of variance < 3%) into 10 dosecalibration points: 10, 20, 30, 50, 100, 200, 300, 400, 500, and800 cGy. After analyzing the TLD readout values, we used thedetermination coefficient (𝑅2) of the linear fitting curve toassess the linearity of the dose-response curve. An 𝑅2 valuegreater than 0.99 was considered to indicate that the formulawas sufficiently accurate for converting the TLD responseinto the actual radiation dose.

BioMed Research International 3

2.3. Determination of Absorbed Dose in Water. A TLD with aresponse uncertainty of within 3%was used in this study.Theabsorbed dose in water for high-energy photon and electronbeams was obtained using the following equation:

Dose (TLD, 60Co) = 𝑅 × CF × 𝑓𝑒× 𝑓𝑝× 𝑓𝑠, (1)

where

Dose is the absorbed dose in water for high-energybeams,𝑅 is the average response of the TLD,CF is the dose-response conversion factor,𝑓𝑒is the energy-dependence correction factor,𝑓𝑝is the phantom scatter correction factor,𝑓𝑠is the setup-condition factor.

2.3.1. Energy-Dependence Correction Factor (𝑓𝑒). In verifying

the dose of radiation therapy, the main objective was toconfirm the radiation doses commonly used in radiotherapyunits, such as 6 and 10MV photons and 6, 9, and 12-MeV electron beams. Because the TLD responses differsomewhat between high-energy beams and 60Co, the energy-dependence correction factor (𝑓

𝑒) should be utilized in

determinations of the absorbed dose in water. To obtain 𝑓𝑒

for each phantom, we first used an ion chamber (WellhoferFC65-P) in accordance with the AAPM TG-21 protocol tocalibrate the beam output of the Varian LINAC using the air-kerma calibration factor (𝑁

𝑘= 4.4482 × 10

7 Gy/C) providedin the INER ion-chamber dose calibration report. Afterirradiating the TLDs with a known dose using high-energybeams and 60Co, the response to the unit dose representedthe sensitivity, and this value was normalized to that for 60Coas the energy-dependence correction factor according to

𝑓𝑒= (TLD response of unit dose from Co-60 source)

× (TLD response of unit dose

from high-energy beams)−1 .

(2)

2.3.2. Phantom Scatter Correction Factor (𝑓𝑝). The size of

a phantom influences the scattering dose and the charged-particle equilibrium. Correcting for the scattering dosebetween these phantoms and a water phantom is necessary.Under the condition of fixed distances between the sourceand the chamber for the photon and electron beams, we usedthe ion chamber tomeasure the ionization ratio of these smallacrylic phantoms and the full-scatter solid water (30 × 30 ×30 cm3) for high-energy beams: 6 and 10MV photon beamsand 6, 9, and 12MeV electron beams. According to AAPMTG-21, the absorbed dose in medium is defined as

𝐷med (𝑑max) = 𝑀corrected𝑁gas𝑃repl𝑃wall (𝐿

𝜌

)

med

gas. (3)

Because the same ionization chamber was used to mea-sure the 𝐷med values of acrylic and water, the 𝑁gas, 𝑃repl,

and 𝑃wall parameters are the same in these two mediums.Therefore, the dose ratio for the solid water and postalphantoms was defined as the phantom scatter correctionfactor, calculated using the ionization ratio and the ratio ofthe mean restricted mass collision stopping power:

phantom scatter correction factor

=

𝐷water𝐷arcylic=

𝑀water𝑀arcylic(

𝐿

𝜌

)

water

arcylic.

(4)

Considering the phantom stability, we also used threedifferent types of LINACs (produced by Varian, Siemens, andElekta) and irradiated the measuring point of each phantomat 100 MU as calculated in the reference condition.

2.3.3. Setup-Condition Factor (𝑓𝑠). The dose calibration con-

ditions differ for each LINAC when using photon and elec-tron beams. These following reference points are commonlyused: (1) photon beams-SSD= 100 cm, depth = 𝑑max or SAD=100 cm, depth = 𝑑max or SSD = 100 cm, depth = 5 cm; and (2)electron beams-SSD = 100 cm, depth = 𝑑max.

Therefore, the factor used to convert the dose in amailable phantom varies with the dose calibration conditionsat each hospital. The setup-condition factors calculated forthe beam data included the tissue-air ratio (TAR), tissue-phantom ratio (TPR), percentage depth dose, and inverse-square law.

As an example, for 6MV photon beams, the dose refer-ence point is represented as𝐷

𝐸(SAD= 100 cm, depth= 5 cm),

which means that 1 MU is equal to 1 cGy. Referring to (5),after applying the phantom scatter correction factor in (1), thedose at a particular depth in water is𝐷

𝐴. Then, the TAR and

inverse-square law could be used to convert𝐷𝐴into𝐷

𝐵,𝐷𝐶,

and𝐷𝐷. Finally, according to the clinical units providing TPR

value, the𝐷𝐷was converted to𝐷

𝐸(shown in Figure 2):

𝐷𝐵

𝐷𝐴

=

1

TAR (𝑑 = 1.5 cm, field size = 9.85 × 9.85 cm2),

𝐷𝐶

𝐷𝐵

= (

98.5

100

)

2

,

𝐷𝐷

𝐷𝐶

= TAR (𝑑 = 1.5 cm, field size = 10 × 10 cm2) ,

𝐷𝐸

𝐷𝐷

=

1

TPR (𝑑 = 1.5 cm, field size = 10 × 10 cm2).

(5)

3. Results

3.1. TLD Calibration. The above-described dose calibrationprocedures allowed the TLD response to be analyzed for10 dose points. The linear trend line equation was usedto convert the TLD response to the radiation dose. Afterchecking the feasibility, as indicated by𝑅2 exceeding 0.99, theirradiated and calculated doses for the TLDs were confirmedas being linearly dependent (Figure 3).

4 BioMed Research International

5cm

1.5 cm

100

cm 98.5 cm

DA DB DC DD DE

Figure 2: The schematic of setup condition.

0

100

200

300

400

500

600

700

800

900

0 100 200 300 400 500 600 700 800 900Dose (irradiated, cGy)

Dos

e (ca

lcul

ated

, cG

y)

y = 0.9984x + 0.3357

R2 = 0.9984

Figure 3: The dose linearity of TLD (calculated dose versusirradiated dose).

3.2. Energy-Dependence Correction Factor. The energydependence of high-energy beams compared with the 60Cosource at the INER is shown in Figure 4. The 𝑓

𝑒values

were 1.005 and 1.003 for the 6 and 10-MV photon beams,respectively, and 1.006, 0.991, and 0.993 for the 6, 9, and12-MeV electron beams. The standard deviation of 𝑓

𝑒was

0.73%, with no apparent variance.

3.3. Phantom Scatter Correction Factor. The phantom scattercorrection factors of the homemade self-designed phantomsfor each beam energy produced by the five LINACs are shownin Figure 5. For the homemade phantoms, this factor wasstable and near to 1 for each of the high-energy beams.The results for the energy-dependence correction factorsindicated that the properties of the TLD-100 were stable.Thisreflects the small variance in the interactionwith high-energybeams due to the lower atomic number.

In order to check the validity of the audit proceduresand the relative parameters, we used the ion chamber andfollowed AAPM TG-21 to measure the daily output dose.TLDs combined with two types of phantoms were exposed to200MU.Applying (1), the TLD response was converted to theabsorbed dose in water with full scatter. The doses measuredby the ionization chamber and TLDs are compared in Table 1.Most of the differences between the beams in the phantomsare within ±3%, which confirms the accuracy and stability ofthe phantom designed in this study.

0.97

0.98

0.99

1

1.01

1.02

1.03

Co-60 6x 10x 6e 9e 12eBeam energy

Cor

rect

ion

fact

or

Figure 4: The energy-dependence correction factor of TLD.

0.94

0.96

0.98

1

1.02

1.04

1.06

1.08

6x 10x 6e 9e 12eBeam energy

Scat

ter f

acto

r

Figure 5: Scatter correction factor of phantom.

Table 1: The dose comparison between ionization chamber andTLDs.

𝐷TLD (cGy) 𝐷IC (cGy) Difference (%)6MV photon 194.48 ± 2.32 199.50 ± 1.92 −2.519%10MV photon 197.21 ± 3.93 197.97 ± 1.87 −0.386%6MeV electron 203.71 ± 2.04 199.32 ± 1.56 2.205%9MeV electron 194.84 ± 3.62 198.45 ± 2.03 −1.819%12MeV electron 198.99 ± 3.13 198.04 ± 1.58 0.478%𝐷TLD: dose measured from TLDs; 𝐷IC: dose measured from the ionizationchamber.

4. Conclusion

Radiation-therapy institutions worldwide participate inpostal audit programs. This study investigated this processbased on consideration of the procedures of phantom setup,dose delivery, and the formula for the absorbed dose inwater. The derived equations facilitated the easy comparisonof the reproducibility and accuracy of TLD systems. Thestable TLDs were then used in a dose-verification system,and the dose calibration curve was found to be suitable forconverting the TLD readout value into an accurate dose. Aftercomparing with the measurements made using an ionizationchamber and following AAPM TG-21, the analogous doses

BioMed Research International 5

calculated from TLDs indicated the validity of the correctionfactors in (1). The light weight, full scattering dose, anddepth-adjusting features of the designed phantoms allowthem to be used to verify other medical-physics parameterssuch as the beam quality, output factors, and wedge factors.

Powder TLDs used by the IAEA in postal audit programshave some advantages, such as being waterproof and highlyaccurate, but the present study has demonstrated that cubicTLDs provide more convenient measurements with a lowworkload.The designed phantoms and operating proceduresreported herein can be used to verify the doses applied inradiation-therapy institutions.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgments

This study was supported in part by the Ministry of Scienceand Technology of Taiwan (MOST 102-2623-E-010-003-NU)and by the Atomic Energy Council of Taiwan (NL1030792).

References

[1] J. Izewska and P. Andreo, “The IAEA/WHO TLD postal pro-gramme for radiotherapy hospitals,” Radiotherapy and Oncol-ogy, vol. 54, no. 1, pp. 65–72, 2000.

[2] J. Izewska, P. Andreo, S. Vatnitsky, and K. R. Shortt, “TheIAEA/WHO TLD postal dose quality audits for radiotherapy:a perspective of dosimetry practices at hospitals in developingcountries,” Radiotherapy & Oncology, vol. 69, no. 1, pp. 91–97,2003.

[3] J. Izewska, P. Bera, and S. Vatnitsky, “IAEA/WHO TLD postaldose audit service and high precision measurements for radio-therapy level dosimetry,” Radiation Protection Dosimetry, vol.101, no. 1–4, pp. 387–392, 2002.

[4] F. Pernicka, A. Meghzifene, H. Tolli, and J. Izewska, “IAEATLD based audits for radiation protection calibrations at sec-ondary standards dosimetry laboratories,” Radiation ProtectionDosimetry, vol. 101, no. 1–4, pp. 275–278, 2002.

[5] J. Izewska, D. Georg, P. Bera et al., “A methodology for TLDpostal dosimetry audit of high-energy radiotherapy photonbeams in non-reference conditions,” Radiotherapy and Oncol-ogy, vol. 84, no. 1, pp. 67–74, 2007.

[6] I.H. Ferreira, A.Dutreix, A. Bridier, J. Chavaudra, andH. Svens-son, “The ESTRO-QUALity assurance network (EQUAL),”Radiotherapy and Oncology, vol. 55, no. 3, pp. 273–284, 2000.

[7] D. S. Followill, D. A. R. Evans, C. Cherry et al., “Design,development, and implementation of the radiological physicscenter’s pelvis and thorax anthropomorphic quality assurancephantoms,”Medical Physics, vol. 34, no. 6, pp. 2070–2076, 2007.

[8] A. Bridier, H. Nystrom, I. Ferreira, I. Gomola, andD. Huyskens,“A comparative description of three multipurpose phantoms(MPP) for external audits of photon beams in radiotherapy: thewater MPP, the UmeaMPP and the ECMPP,” Radiotherapy andOncology, vol. 55, no. 3, pp. 285–293, 2000.

[9] D. Kroutilıkova, J. Novotny, and L. Judas, “Thermoluminescentdosimeters (TLD) quality assurance network in the Czech

Republic,” Radiotherapy and Oncology, vol. 66, no. 2, pp. 235–244, 2003.

[10] S. E. Davidson, R. A. Popple, G. S. Ibbott, and D. S. Followill,“Technical note: heterogeneity dose calculation accuracy inIMRT: study of five commercial treatment planning systemsusing an anthropomorphic thorax phantom,” Medical Physics,vol. 35, no. 12, pp. 5434–5439, 2008.

[11] Y. Han, E. H. Shin, C. Lim et al., “Dosimetry in an IMRTphantom designed for a remote monitoring program,” MedicalPhysics, vol. 35, no. 6, pp. 2519–2527, 2008.

[12] A. Molineu, N. Hernandez, T. Nguyen, G. Ibbott, and D.Followill, “Credentialing results from IMRT irradiations of ananthropomorphic head and neck phantom,” Medical Physics,vol. 40, no. 2, Article ID 022101, 2013.

[13] T. Kron, A. Haworth, and I. Williams, “Dosimetry for audit andclinical trials: challenges and requirements,” Journal of Physics:Conference Series, vol. 444, no. 1, Article ID 012014, 2013.

[14] G. S. Ibbott, “QA in radiation therapy: the RPC perspective,”Journal of Physics: Conference Series, vol. 250, Article ID 012001,2010.

Submit your manuscripts athttp://www.hindawi.com

Stem CellsInternational

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

MEDIATORSINFLAMMATION

of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Behavioural Neurology

EndocrinologyInternational Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Disease Markers

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

BioMed Research International

OncologyJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Oxidative Medicine and Cellular Longevity

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

PPAR Research

The Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014

Immunology ResearchHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Journal of

ObesityJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Computational and Mathematical Methods in Medicine

OphthalmologyJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Diabetes ResearchJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Research and TreatmentAIDS

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Gastroenterology Research and Practice

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Parkinson’s Disease

Evidence-Based Complementary and Alternative Medicine

Volume 2014Hindawi Publishing Corporationhttp://www.hindawi.com